An intrinsic semiconductor possesses relatively poor conductivity. The vast majority of the electrons in an intrinsic semiconductor are in the valence band whereas relatively few electrons are in the conduction band. The band gap between the valence and conduction bands presents an energy barrier which inhibits the movement of electrons from the valence band to the conduction band. When exposed to light, however, electrons may be excited from the valence band to the conduction band. The promoted electron and the resulting positive charge left behind in the valence band form an electron-hole pair (EHP). As more and more EHPs are created, the conductivity will increase substantially. Solid-state optical detectors (photodetectors) exploit this increase in conductivity, denoted as photoconductivity, to detect and/or measure optical excitation. Photodiodes form a particularly useful group of solid-state optical detectors, providing a rapid and sensitive response to optical or high-energy radiation.
In contrast to other types of solid-state optical devices, photodiodes possess a single p-n junction. Should a photodiode be used as a photodetector, the p-n junction is typically reverse biased, thereby forming a relatively wide depletion region. The current through the reverse-biased p-n junction will be a function of the optically-generated electron-hole pairs. Carriers resulting from an electron-hole pair formation in the depletion region are swept out by the electric field resulting from the reverse bias, thereby providing a rapid response. With the proper doping and reverse biasing, the carriers resulting from an electron-hole formation cause impact ionization of other carriers in a multiplication region, a process denoted as avalanche breakdown. Photodiodes configured for such breakdown are denoted as avalanche photodiodes. Because a single optically-induced carrier may produce many additional avalanche-ionized carriers, the resulting photocurrent gain makes avalanche photodiodes very sensitive and high-speed optical detectors.
The avalanche breakdown process, being stochastic, is inherently noisy. To minimize this noise, the doping profile and position of the multiplication region are typically configured to favor the carrier type having a larger ionization efficiency. Another problem avalanche photodiodes must minimize is the dark current, which is the generation current across the p-n junction resulting from thermally-created carriers in the absence of any light excitation. As implied by the name, separate absorption and multiplication (SAM) avalanche photodiodes separate the absorption region from the multiplication region to reduce the dark current. SAM avalanche photodiodes are typically heterojunction semiconductor devices comprised of different III–V materials such as InGaAs and InP. A low band gap material such as InGaAs is used within the absorption region whereas a high band gap material such as InP forms the multiplication region. Because of this arrangement, the electric field is much higher in the multiplication region as compared to the absorption region. Low band gap materials will typically have greater tunneling current (dark current). However, because the electric field is relatively low across the low band band gap material, tunneling dark current is suppressed.
Regardless of whether the absorption region and the multiplication region are separated, avalanche photodiodes (APDs) may reduce noise by preferentially injecting carriers having a higher ionization efficiency (holes or electrons) into the multiplication region. High-speed photodetectors using APDs require electronic amplifiers that set the noise floor and limit sensitivity. The current gain provided by APDs alleviates the sensitivity limitation. However, the noise suffers if the APD capacitance and/or the dark current are too large.
One approach to minimize APD capacitance is to reduce the device area. Each pixel in an APD array must be isolated from adjacent pixels such that there is a fixed area around each pixel that cannot detect light. To minimize the dead space between pixels, it is desirable to increase the area that collects and amplifies the light signal. However, as the pixel size is decreased to reduce APD capacitance, the “fill factor” or ratio of light receiving to total-array size decreases. To prevent this undesirable tradeoff, a “microlens” may be used to focus light from a relatively large area such as 100×100 um onto a relatively small area such as 20×20 um to achieve a high fill factor while having a small pixel dimension. However, such an approach requires significant additional processing on the backside of a fragile semiconductor wafer as well as demanding relatively tight alignment tolerances between the front side APD pixel pattern and the backside microlens pattern.
Accordingly, there is a need in the art for avalanche photodiodes having reduced capacitance.